Add option to generator for skippable fields

[jhorstmann]

• Add a configuration object to any struct::read or fill methods, with a boolean per skippable field
• Adjust generated match statements, for example

match last_field_id {
    ...
    3 => if !config.skip_meta_data {
        self.meta_data.fill_field(input, field_type)?;
    }

[jhorstmann]

The config would have to be a generic parameter on the CompactThriftInput trait. That is a bit annoying since that parameter would be unused by all the implementations on standard types, and would cause additional code generation during monomorphization for different config types.